Control of collective effects in chiral active colloidal systems*

Chiral active systems represent a class of active matter systems where the constituent particles break time reversal symmetry by orbiting or spinning, rather than self-propulsion. Similar to the emergence of flocking states in self-propelled systems, novel collective effects such as active crystallization and circulating clusters have been observed in biological chiral active systems. Here we experimentally study chiral colloidal systems that are driven by external fields. We show that in a system of spinning colloidal magnets driven by a rotational magnetic field the system phase separates into circulating clusters with unidirectional edge flows. The emergent behavior is sensitive to the particle shape and by using particles of different geometries we demonstrate the formation of different collective phases. In addition to the hydrodynamic interactions between spinning particle, we introduce diffusiophoretic interactions by suspending the hematite colloids in H₂O₂ where they generate phoretic flows in the presence of UV light. The added interfacial flows lead to the formation of bound states between spinning colloids that are stabilized through near-field hydrodynamic and chemical interactions and at a collective level cause a loss in structural cohesion of the circulating clusters. Finally, we use two-photon-polymerization lithography to fabricate geometrically anisotropic colloids that exhibit controllable chiral motion in uniform electric fields, even in the absence of any external rotating field.